The standard method of test for adhesion of vulcanized rubber to wire in use at the present time is set forth in ASTM standard designation D1871-68 with modifications according to ASTM D2229-73. This test is referred to herein as the Standard Block Adhesion Test or SBAT. These standard tests now in use have long been known not to be fully satisfactory, one objection being the wide scatter of measured results obtained in tests of seemingly identical samples, and a second objection being their inability to distinguish the effect of intrinsic adhesion from the effects of rubber stiffness and rubber tearing strength. We have discovered that some of the disadvantages are related to the fact that the wire or wires extend continuously through the body of the sample and that difficulties experienced are also associated with the fact that the elastomeric body of the sample must be restrained or confined to resist the pull exerted on the wire of the specimen.
We have also discovered by the use of photoelastic techniques that to overcome the objections and disadvantages of the prior testing for adhesion that the end of the wire of the specimen must be embedded within the body of the specimen as distinguished from extending through the body. We have further discovered that the body is advantageously subjected only to the pull of the wire and not to any external restraint and, further, that this can be accomplished conveniently by embedding two collinear wire segments the proximate ends of which are spaced apart not less than a predetermined minimum distance within the body of the specimen.
It is, therefore, the object of the invention to provide a method and apparatus for testing the intrinsic interface adhesion between wire and rubber improved with respect to the methods and apparatus heretofore in use.
Broadly, the invention provides a method of testing interface adhesion of wire and rubber comprising:
preparing two wire segments each of predetermined length;
preforming two coupons of uncured rubber to provide for a fresh, uncontaminated face and a predetermined volume;
placing said wire segments, each with a length portion thereof in seated contact with said fresh face of one of said preformed coupons and the remaining length portions thereof extending freely outward lengthwise of such coupon along a single straight line and with the proximate ends of the segments spaced apart along said line, then placing said fresh face of the other of said coupons in face to face contact with the fresh face of said one thereof and thereby embedding the first said length of each said segment and forming a unitary body of the pair of coupons with the proximate ends of two wire segments embedded in and spaced apart in such body a predetermined minimum distance;
pulling said wire segments apart at a predetermined rate while maintaining said body free of restraint;
measuring the force required in said pulling, the force so measured being useful to determine, together with otherwise known rubber stiffness, interface adhesion of said wire and rubber.
Further, the invention provides a specimen molding apparatus for the wire to rubber adhesion test comprising means for preforming a pair of coupons of uncured rubber each having a fresh uncontaminated uncured surface and predetermined volume;
means for seating a pair of wire segments in straight line spaced apart array in one of said fresh surfaces;
mold means for matingly juxtaposing said surfaces with said wire segments there between to form the uncured specimen;
means for compressing said uncured specimen while curing the same; and
vent means for controlling flow of said rubber in and from said specimen during said curing.
To acquaint persons skilled in the arts most closely related to the present invention, certain preferred embodiments thereof illustrating a best mode now contemplated for putting the invention into practice are described herein by and with reference to the annexed drawings forming a part of the specification. The embodiments shown and described herein are illustrative and as will become apparent to those skilled in these arts can be modified in numerous ways within the spirit and scope of the invention defined in the claims hereof.